High quality porous pads are used for filtration and in a number of electronic devices such as batteries and fuel cells. In such devices, the porous pads advantageously allow gases or components dissolved in liquids to pass through. Porous pads are made of micro-fibers, nanofibers, and micro-porous films. Fibers of these dimensions are prepared by electrospinning in the case of solvent soluble polymers. However, polyolefins are difficult to form solutions without maintaining high temperatures in high-boiling solvents. Porous polyolefins are made by biaxial tension on films or sheets of these plastic polymers. Alternatively, pore formers are added to the polyolefin sheets during the fabrication process which are then extracted by solvents or removed with heat. Electrospinning can be used in the case of solvent soluble olefins which can be processed in solutions.
In proton exchange membrane (“PEM”) type fuel cells, hydrogen is supplied to the anode as fuel, and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. Typically, the ion conductive polymer membrane includes a perfluorosulfonic acid (“PFSA”) ionomer.
Each catalyst layer has finely divided catalyst particles (for example, platinum particles), supported on carbon particles, to promote oxidation of hydrogen at the anode, and reduction of oxygen at the cathode. Protons flow from the anode through the ion conductive polymer membrane to the cathode where they combine with oxygen to form water which is discharged from the cell.
The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”), which in turn are sandwiched between a pair of electrically conductive elements or plates. The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cells in stacks in order to provide high levels of electrical power.
In many fuel cell applications, electrode layers are formed from ink compositions that include a precious metal and a perfluorosulfonic acid polymer (“PFSA”). For example, PFSA is typically added to the Pt/C catalyst ink in electrode layer fabrication of proton exchange membrane fuel cells to provide proton conduction, to the dispersed Pt-carbon catalyst nanoparticles, as well as binding of the porous carbon network. Traditional fuel cell catalysts combine carbon black with platinum deposits on the surface of the carbon, along with ionomers. The carbon black provides (in part) a high surface area conductive substrate. The platinum deposits provide a catalytic behavior, and the ionomers provide a proton conductive component. The electrode is formed from an ink that contains the carbon black catalyst and the ionomer, which combine on drying to form an electrode layer.
Gas diffusion layers have a multifunctional role in fuel cell technology. For example, gas diffusion layers act as diffusers for reactant gases traveling to the anode and the cathode layers while transporting product water to the flow field. Gas diffusion layers also conduct electrons and transfer heat generated at the membrane electrode assembly to the coolant, and act as a buffer layer between the soft membrane electrode assembly and the stiff bipolar plates. Although the present technologies for making gas diffusion layers for fuel cell applications work reasonably well, improvement in properties and cost are still desirable.
Accordingly, the present invention provides improved methods of making porous pads that are useful in fuel cell applications.